To process any events, it needs to find the thread in the machine first.
The machine maintains a rb tree to store all threads. The rb tree is
protected by a rw lock.

It is not a problem for current perf which serially processing events.
However, it will have scalability performance issue to process events in
parallel, especially on a heavy load system which have many threads.

Introduce a hashtable to divide the big rb tree into many samll rb tree
for threads. The index is thread id % hashtable size. It can reduce the
lock contention.

Committer notes:

Renamed some variables and function names to reduce semantic confusion:

  'struct threads' pointers: thread -> threads
  threads hastable index: tid -> hash_bucket
  struct threads *machine__thread() -> machine__threads()
  Cast tid to (unsigned int) to handle -1 in machine__threads() (Kan Liang)
Signed-off-by: NKan Liang <kan.liang@intel.com>
Tested-by: NArnaldo Carvalho de Melo <acme@redhat.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Lukasz Odzioba <lukasz.odzioba@intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1505096603-215017-2-git-send-email-kan.liang@intel.comSigned-off-by: NArnaldo Carvalho de Melo <acme@redhat.com>